The Prime Minister has said he fears the worst for five Britains who are unaccounted for after the Algerian hostage crisis drew to a bloody close.

The Algerian Interior Ministry has said that 23 hostages were killed during the four-day stand-off with the al Qaeda-linked Islamist gunmen who stormed the gas plant in the Saharan desert on Wednesday.

Some 32 militants were also killed, while 107 foreign workers and 685 local employees had been released.

Mr Hague announced the latest casualties after Algerian special forces launched a "final assault" against the terrorists.

Algerian special forces are said by the state news agency, APS, to have stormed the natural gas installation in the middle of the Sahara desert. They reportedly killed 11 militants holding out inside the plant.

Algerian private TV has shown pictures it says came from an escaped hostage

APS quoted an Algerian security source as saying that the hostages who died were killed by their captor.

Algerian troops are understood to be working to remove explosives from the plant, which had been set by the militants as booby traps before the final battle.

Video:Algeria: How The Hostages Survived

It has been reported that troops recovered six machine guns, 21 rifles, two shotguns, two 60mm mortars with shells, six 60mm missiles with launchers, two rocket-propelled grenades with eight rockets and 10 grenades in explosive belts.

Mr Hague said: "We are working hard to get definitive information about each individual. We are in touch with all of the families concerned."

David Cameron said that he had spoken to Algerian prime minister Abdelmalek Sellal, who confirmed that the four-day hostage crisis was "effectively ended".

He said in a statement: "Let me be clear. There is no justification for taking innocent life in this way.

"Our determination is stronger than ever to work with allies right around the world to root out and defeat this terrorist scourge and those who encourage it."

Video:British Hostage: 'We Will Return'

BP chief executive Bob Dudley said 18 of its workers were at the site at the time of the rebel attack and four were still unaccounted for.

Mr Hammond, speaking during a news conference with US counterpart Leon Panetta, said: "The hostage situation has been brought to an end by a further assault by Algerian forces which has resulted in further loss of life."

He said the loss of life was "appalling and unacceptable and we must be clear that it is the terrorists who bear sole responsibility for it".

A consular team is now on the ground to help the Britons who have escaped and ambassador Martyn Roper is in the town of In Amenas near the gas complex.

Video:2013: How Algeria Crisis Unfolded

The militants, under the control of the al Qaeda-linked terrorist Mokhtar Belmokhtar, stormed the plant early on Wednesday morning.

Some of the workers are said to have avoided capture by hiding in the ceiling of living quarters for two days. Others crawled across the desert to make their escape.